1. Field of the Invention
The present invention relates to methods for manufacturing optical components having gradient refractive index distribution, particularly gradient refractive index (GRIN) lenses and GRIN optical fiber preforms.
2. Description of Related Art
Nowadays, the most popular gradient refractive index (GRIN) optical components include axial and radial GRIN lenses and GRIN optical fibers. Axial GRIN lenses are usually used to adjust spherical aberration and simplify optical systems. Radial GRIN lenses have more advantages than axial GRIN lenses. For example, many radial GRIN lenses are self-focused. Therefore such lenses are widely used in optical communications components such as circulators, couplers, switches, dense wavelength-division multiplexers (DWDMs) and light emitting diodes (LEDs). GRIN optical fibers are now widely used in long and short distance information transmission.
Gradient refractive index optical components such as GRIN lenses and optical fibers are presently manufactured by any of a variety of methods including ion exchange, Sol-Gel, bulk diffusion and chemical vapor deposition (CVD).
U.S. Pat. No. 4,902,330 discloses an ion exchange method. This is now the most popular method for producing gradient refractive index lenses. The method includes a first step in which a glass body is immersed into a molten salt. The molten salt contains ions which provide a refractive index higher than that of ions constituting the glass body. This results in ion diffusion into the glass body. In a second step, the glass body is then immersed into another molten salt. This other molten salt contains ions which provide a refractive index lower than that of the ions of the molten salt used in the first step. Thus a predetermined refractive index distribution in the glass body is obtained.
Although the ion exchange method is a comparatively simple and established technique for producing GRIN lenses, it still has several disadvantages. Some of these disadvantages are inherent in the ion exchange mechanism itself. The molten salt ions cannot travel very far into the glass rod to exchange with corresponding outgoing ions. Thus it is almost impossible to make large components, or components with a high refractive index gradient. In addition, products made by this method frequently do not have uniform quality. A proportion of poor quality products is relatively high, which results in high production costs.
U.S. Pat. No. 5,294,573 discloses a so-called Sol-Gel method for making glass gradient refractive index components. The process is initiated by forming a mixture of silicon alkoxide and an alcohol in a solution sufficiently acidic to partially hydrolyze the silicon alkoxide. An index modifying metal alkoxide, such as titanium and zirconium, is then added to the mixture. Water is then added to convert the metal alkoxide to a network of corresponding metal oxides suitable for gelation. The mixture containing the network of metal oxides is then maintained for a sufficient time to form a gel. The gel is acid leached until some of the index modifying metal oxides are removed. The gel is then fixed, to prevent further removal of index modifying metal oxides from the gel. The fixed gel is then rinsed with a solvent to remove precipitates from the gel, then dried, and finally sintered into a transparent gradient refractive index glass. In another version of this method, the fixing agent is acetone or a mixture of water and acetone. Generally this method employs a gel, which allows the metal salts used to modify the refractive index to travel easily. The gel is finally sintered to become a transparent glass rod. Thus the difficulties of ion dispersion of the ion exchange method are circumvented. However, other difficulties arise as a result. The sintered glass rod is often more brittle and less transparent than that of the ion exchange method. In addition, a processing cycle may be as long as 7 to 10 days.
Certain bulk diffusion methods are disclosed in U.S. Pats. Nos. 5,200,858, 5,236,468, 5,917,105 and 5,689,374. Layers of glass plates having different refractive indices and composition are stacked. Thus discontinuous gradient refractive index distribution can be attained. A highly controlled thermal treatment is used to blur the interfaces in the stack, and smooth the previously sharp gradient curve. Large-dimension components with selectable refractive index gradients can be attained using this method, and the initial refractive index distribution is easily controlled. Various kinds of optical glass and even optical polymer material can be used to produce GRIN lenses. Unfortunately, this method can only be used to manufacture axial GRIN lenses. The method cannot be used to manufacture the more popular radial GRIN lenses.
The CVD method includes the following main steps. Accompanying chemical reactions, chemical vapor with continuously changing composition is deposited on tube-shaped or plate-shaped substrates layer by layer. The substrates are then sintered to form a transparent glass rod that has a pre-determined refractive index distribution. This technique can be precisely controlled. However, it is not easy to perform, and requires a long production cycle.
In summary, the above-described methods have a variety of shortcomings. Some require lengthy and costly treatment using items such as special ovens and preparations of powders, liquids and gases. Others are unable to produce large components, or components that have a high refractive index gradient.
In view of the above, an object of the present invention is to provide a simple method for efficiently mass producing gradient rods and optical fiber preforms, and thereby minimize the cost of gradient refractive index components.
Another object of the present invention is to provide a method for producing radial gradient refractive index optical components of any desired dimension and having any desired refractive index gradient.
A further object of the present invention is to avoid the need for lengthy inter-diffusion, chemical or thermal treatments in the manufacture of gradient refractive index (GRIN) lenses or fiber preforms.
A still further object of the present invention is to provide means and a method for accurate control of gradient refractive index profiles in gradient refractive index optical components.
A method for making gradient refractive index optical components in accordance with the present invention includes mixing a molten basic material with a refractive index modifying material in continuously changing proportions. The mixture is changed into a plurality of semi-molten fibers, and the fibers are rolled to form a continuous plate. The plate has a continuously changing refractive index along a lengthwise direction thereof. The plate is wound around a spindle to obtain a wound preformed rod. The preformed rod is locally heated to integrally fuse it, and drawn to form a draw having a predetermined diameter. The draw is cut into pieces. Each piece can then be made into an optical component having a continuously changing refractive index in a radial direction.
The method of the present invention allows precise control of all steps. In addition, the control is achieved with relative ease throughout. The method can be used for the manufacture of both radial and axial GRIN components. Furthermore, the method can be used for the manufacture of large sized optical components, and of optical components having a large refractive index distribution.
Other objects and advantages of the present invention will become apparent from the following description.